Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from flat pieces of material that are substantially transparent, such as quartz or glass, with an opaque layer, such as chrome, on one side. Etched in the opaque layer (e.g., chrome) of the mask is a pattern corresponding to a portion of an electronic circuit design. A variety of different photomasks, including for example, aaPSMs, embedded attenuated phase shift masks and binary photomasks (e.g., chrome-on-glass), are used in semiconductor processing to transfer these patterns onto a semiconductor wafer or other type of wafer.
As shown in FIGS. 1a and 1b, to create an image on a semiconductor wafer 20, a photomask 9 is interposed between the semiconductor wafer 20 (which includes a layer of photosensitive material) and an optical system 22. Energy generated by an energy source 23, commonly referred to as a Stepper, is inhibited from passing through opaque areas of the photomask 9. Likewise, energy from the Stepper passes through the substantially transparent portions of the photomask 9, thereby projecting a diffraction limited, latent image of the pattern on the photomask onto the semiconductor wafer 20. In this regard, the energy generated by the Stepper causes a reaction in the photosensitive material on the semiconductor wafer such that the solubility of the photosensitive material is changed in areas exposed to the energy. Thereafter, the photosensitive material (either exposed or unexposed) is removed from the semiconductor wafer 20, depending upon the type of photolithographic process being used. For example, where a positive photolithographic process is implemented, the exposed photosensitive material becomes soluble and is removed. By contrast, where a negative photolithographic process is used, the exposed photosensitive material becomes insoluble and the unexposed, soluble photosensitive material is removed. After the appropriate photosensitive material is removed, a pattern corresponding to the photomask 9 appears on the semiconductor wafer 20. Thereafter, the semiconductor wafer 20 can be used for deposition, etching, and/or ion implantation processes in any combination to form an integrated circuit.
As circuit designs have become increasingly complex, semiconductor manufacturing processes have become more sophisticated to meet the requirements of these complexities. In this regard, devices on semiconductor wafers have continued to shrink while circuit densities have continued to increase. This has resulted in an increased use of devices packed with smaller feature sizes, narrower widths and decreased spacing between interconnecting lines. For photolithographic processes, resolution and depth of focus (DoF) are important parameters in obtaining high fidelity of pattern reproduction from a photomask to a wafer. However, as feature sizes continue to decrease, the devices' sensitivity to the varying exposure tool wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.) used to write images on a semiconductor wafer has increased, thereby making it more and more difficult to write to an accurate image on the semiconductor wafer. In this regard, as feature sizes continue to decrease, light diffraction effects in the photomask are exacerbated, thereby increasing the likelihood that defects will manifest in a pattern written on a semiconductor wafer. Accordingly, it has become necessary to develop new methods to minimize the problems associated with these smaller feature sizes.
One known method for increasing resolution in smaller feature sizes involves the use of shorter exposure wavelengths (e.g., 248 nm, 193 nm, 157 nm, 13 nm, etc.). Shorter exposure wavelengths, however, typically result in a shallower DoF in conventional binary chrome-on-glass (COG) photomasks having smaller feature sizes. In this regard, when the feature size is smaller than the exposure tool wavelength, binary COG photomasks become diffraction limited, thereby making it difficult, if not impossible, to write an accurate image on the semiconductor wafer. Accordingly, phase shifting masks (“PSMs”) have been used to overcome this problem. In this regard, PSMs are known to have properties which permit high resolution while maintaining a sufficient DoF. More particularly, a PSM reduces the diffraction limitation ordinarily associated with a binary COG mask by passing light through substantially transparent areas (e.g., glass or quartz) which have either different thickness and/or different refractive indices than an ordinary binary COG mask. As a result, destructive interference is created in regions on the target semiconductor wafer that are designed to see no exposure. Thus, by reducing the impact of diffraction through phase shifting, the overall printability of an image is vastly improved such that the minimum width of a pattern resolved by using a PSM is approximately half the width of a pattern resolved in using an ordinary binary COG mask.
Various types of PSMs have been developed and are known in the art, including aaPSMs. FIGS. 2a-b illustrate an example of a conventional aaPSM 10. An aaPSM is typically comprised of a layer of opaque material and a substantially transparent substrate which is etched on one side of the opaque features, while not etched on the other side (i.e., etching of the transparent substrate occurs in alternating locations in the substantially transparent substrate). More particularly, as shown in FIGS. 2a-b, the aaPSM 10 includes a substantially transparent layer (e.g., quartz) and an opaque layer (e.g., chrome). The opaque layer is etched to form opaque regions 15 and alternating substantially transparent regions 13, as shown in FIG. 2b. The substantially transparent regions 13 are further etched such that the aaPSM 10 has recesses 14 in the substantially transparent layer. In other words, the aaPSM 10 has substantially transparent regions 13 (which are un-etched) that alternate with etched recesses 14 between each opaque region 15, as shown in FIGS. 2a-b. The effect of this structure when placed in a Stepper is to create light intensity of alternating polarity and 180° out of phase, as shown in FIG. 2c. This alternating polarity forces energy transmitted from the Stepper to go to zero, in theory, at opaque regions 15 while maintaining the same transmission of light at the alternating transparent regions 13 and recesses 14. As a result, refraction is reduced through this region. In this regard, in recesses 14, the following equation is satisfied:d=λ/2(n−1)where d is film thickness, n is refractive index at exposure wavelength, ÿ is exposure wavelength. Thus, it is possible to etch smaller features in a semiconductor wafer and use shorter exposure wavelengths. Since the photoresist layer on the semiconductor wafer (FIG. 2d) is insensitive to the phase of the exposed light, the positive and negative exposed regions appear the same, while the zero region in between is clearly delineated. Thus, a sharper contrast between light (i.e., transparent) and dark (i.e., opaque) regions in the resulting photoresist layer of a semiconductor is obtained, thereby making it possible, in theory, to etch a more accurate image onto the semiconductor wafer.
In practice, however, the aaPSM of FIG. 2b has certain limitations which often preclude the possibility of transferring an accurate image from the aaPSM to a semiconductor wafer. In this regard, as feature sizes continue to get smaller, the intensity of light transmitted through recess 14 will often be less than the intensity of light transmitted through the unetched portions 13. More particularly, referring to FIG. 2e, the light intensity is shown for each transmissive region of the aaPSM of FIGS. 2a and 2b. Length A represents the critical dimensions of recess 14 and Length B represents the critical dimensions of the unetched portion 13 of the substantially transparent layer. As can be seen, the critical dimensions of these features are asymmetrical (Length B-Length A). As a result, the intensity of light transmitted through the unetched portion 13 of the substantially transparent layer is greater than the intensity of light transmitted through the recess 14. This imbalance of light intensity transmitted through these features often make it difficult, if not impossible, to write an accurate image on the a semiconductor wafer using the aaPSM shown in FIG. 2b. 
It is known in the art of photomask design to etch highly anisotropic features (i.e., features etched more in one direction than in other directions) in aaPSMs, as shown in FIGS. 3a and 4a. Anisotropic features are typically formed by using a plasma reactor. In particular, it is known to use a fluorocarbon or hydrofluorocarbon etching gas and apply a radio frequency (“RF”) bias to the pedestal supporting the photomask. The RF bias creates a direct current (“DC”) bias in the plasma adjacent to the mask. The DC bias accelerates the ions towards the mask and the resulting etch is highly anisotropic with nearly vertical sidewalls. In addition to plasma etching techniques, wet etching techniques have been used to undercut features in the phase shift mask, as shown in FIG. 3b. 
However, anisotropic features produce a waveguide effect during wafer printing which induces an aerial image intensity imbalance through focus on the wafer, as shown in FIGS. 3a-3h. For example, as shown in FIGS. 3c, 3e and 3g, aerial image intensity imbalance caused by aaPSM quartz features having sidewalls that have been anisotropically etched can result in a relative difference of exposure intensity at the wafer plane if the stepper is not in perfect focus. For example, where the stepper is −0.4 μm out of focus, the aerial image intensity of the energy transmitted through the aaPSM of FIG. 3a is approximately 2.8 a.u. for shallow etched features and 2.2 a.u. for deep etched features, and approximately 3.5 a.u. for shallow etched features and 3.0 a.u. for deep etched features when in perfect focus (i.e., 0.0 μm). Any imbalance in aerial image intensity will result in an inaccurate image being written on the semiconductor wafer. In this regard, since the threshold energy needed to activate photoresist on the wafer is constant, any dissimilarity in intensity for adjacent features will produce a different final critical dimension for adjacent features on the wafer. As a result, the focus latitude required to obtain good pattern transfer from the photomask to the wafer is reduced. This impact on printability due to the waveguide effect has been shown in the prior art to be effectively eliminated by isotropically etching quartz trench features which were formed by anisotropic etching methods.
A known method for reducing aerial image intensity imbalance is to create isotropic trenches in conventional aaPSMs by utilizing: a dry plasma etching step to form an anisotropic trench; and thereafter, a wet hydrofluoric acid (HF) dip, as described in U.S. patent application Publication No. 2001/0044056 A1 to isotropically etch the anisotropic trench. As shown in FIGS. 3a, 3c, 3e and 3g, the aerial image intensity imbalance in this type of aaPSM is significantly reduced when the stepper is out of focus. Although useful for reducing aerial image intensity imbalance, the known methods (e.g., a dry etch followed by a wet etch) has significant drawbacks which have deterred photomask manufacturers from implementing this otherwise useful aaPSM. In particular, HF is known to be a very toxic and corrosive chemical which is hazardous to handle in a production environment. Thus, any alternative method that can achieve the same results without resorting to the use of this hazardous material is preferred. Additionally, HF requires separate processing equipment, and thus, makes the overall manufacture of photomasks more expensive and time consuming. Additionally, since HF is hazardous to the environment, it is necessary to dispose of it in a proper and lawful manner, which can also be costly and burdensome. Furthermore, the wet etch process is purely isotropic in nature and cannot be tuned to prevent excessive undercut and chrome liftoff. Excessive undercut and chrome liftoff is disadvantageous because it can cause defects. Thus, any process which can limit the need for undercutting chrome is preferred, especially where smaller feature sizes are used. An additional concern with respect to wet chemistry is the loading effects of dense to isolated patterned areas. In this regard, an isolated area's etch rates are effected by chemical dilution due to the extreme exposed areas, thereby making it difficult to control the etch time. Thus, wet etching techniques often result in excessive undercut in such exposed areas. Therefore, what is needed is an improved method for manufacturing aaPSMs without an aerial intensity imbalance which avoids using hazardous materials and is tunable to avoid excessive undercut and chrome liftoff and can minimize loading effects.
Although these prior art methods are useful in providing for balanced light intensity for some aaPSM designs, the additional step of undercutting the opaque regions of the photomask is both time-consuming and expensive. Accordingly, the overall number of aaPSMs which could be manufactured in a given time period is limited by these factors. Additionally, as feature sizes continue to get smaller, it will become increasingly difficult to undercut the chrome regions and the problems of chrome liftoff and excessive undercut will become increasingly prevalent. As a result, the use of aaPSMs will become less desirable and potentially obsolete. Moreover, the wet etching techniques of the prior art are known to be hazardous.
Thus, there is a long felt need for a new aaPSM and method for making the same which eliminates the need to undercut the opaque layer while at the same time provides for the transmission of balanced light intensities through the aaPSM.
Accordingly, it is an object of the present invention to provide an improved aaPSM which has a balanced aerial intensity which does not utilize hazardous materials.
It is another object of the present invention to provide an aaPSM for use in photolithography and for semiconductor fabrication to enhance resolution and depth of focus.
It is another object of the present invention to provide an improved aaPSM which has a balanced aerial intensity without excessive undercut and chrome liftoff.
It is another object of the present invention to solve the shortcomings of the prior art.
Other objects will become apparent from the foregoing description.